What is "mi5"

1939, from abbreviation of Military Intelligence, followed by the department number.

The Security Service, also MI5 ( Military Intelligence, Section 5), is the United Kingdom's domestic counter-intelligence and security agency and is part of its intelligence machinery alongside the Secret Intelligence Service (MI6), Government Communications Headquarters (GCHQ) and Defence Intelligence (DI). MI5 is directed by the Joint Intelligence Committee (JIC), and the service is bound by the Security Service Act 1989. The service is directed to protect British parliamentary democracy and economic interests, and counter terrorism and espionage within the UK.

Within the civil service community the service is colloquially known as Box 500 (after its official wartime address of PO Box 500; its current address is PO Box 3255, London SW1P 1AE).

The service has had a national headquarters at Thames House on Millbank in London since 1995, drawing together personnel from a number of locations into a single HQ facility. Thames House was, until March 2013, shared with the Northern Ireland Office (NIO) and is also home to the Joint Terrorism Analysis Centre, a subordinate organisation to the Security Service. The service has offices across the United Kingdom including an HQ in Northern Ireland.

Details of the northern operations centre in Greater Manchester were revealed by the firm who built it. Plans to open the northern operations centre were reported by The Manchester Evening News in February 2005, and plans to open a permanent Scottish office in Glasgow were reported by The Scotsman in January of that year.

The MI5(g), or the MI5 G section, was a branch of MI5 that was formed during World War I to address the war time espionage operation by the Indian revolutionary movement in Europe. The department arose by renaming the MO5(g), which was renamed MI5(g) in 1916. The MI5 itself, working under Vernon Kell, had a number of India experts at the beginning of the war. In September 1916, a special section, the MI5(d), section was formed to operate counter-espionage networks throughout the British Empire. Another subsection, the MI5(b), was formed in January 1917 to deal specifically with Indians and "other oriental races". The MI5(g) had 27 officers in its staff, eight of whom had served in India before the war. Among them were ex-Indian civil servants including Robert Nathan and H. L. Stephenson. The main emphasis of this counter-espionage network was to prevent the subversion of Indian troops in the European theatre. The organisation, especially under Nathan, worked closely with the Special Branch of the Scotland Yard in Britain and with the Indian Political Intelligence Office headed by John Wallinger, which operated a network of spies in neutral Switzerland which a number of the Indian revolutionaries and members of the Berlin Committee used as a base.

MI5(g)'s work at the time identified the plans by Ghadar Party and the Berlin Committee to assassinate Lord Kitchener in 1915 through an associate of Har Dayal, Gobind Behari Lal. It was also responsible at this time, along with Basil Thomson, for the capture of Harish Chandra (who was associated with the Berlin Committee) and turning him into a double agent. Through Harish Chandra was also identified plans for obtaining information of Ghadarite intrigues in Japan and China. Among other works, Nathan was responsible for the plans made by British intelligence in late 1915 to assassinate Virendranath Chattopadhyaya through an agent by the name of Donald Gullick.